ASTR 1230 (O'Connell) Lecture Notes

1. INTRODUCTION TO THE NIGHT SKY AND CONSTELLATIONS

Astronomy is primarily an observational science. It is driven
more by new observational discoveries than by interpretive insights.
Few important astronomical discoveries were predicted, and many were
actually accidental. The human imagination has never been a match
for the universe.
Astronomical discovery began with the simplest of observations: people
looking at the night sky and trying to understand what they were
seeing. In the past, most people were well acquainted with the basic
features of the night sky. We are unfamiliar with the sky in modern
times mainly because of the advent of artificial lighting, which makes
it difficult to see the night sky in urban areas (and also unnecessary
to know the sky as a pathfinder).
This lecture introduces you to the basic features of the night sky which
are visible to the unaided eye and prepares you for the Constellation
Laboratory.

A. NAKED-EYE ASTRONOMY

"Naked eye" observations (i.e. without optical aid from lenses or
mirrors) were the only kind possible for most of human
history! Telescopes were not invented until 1608 AD.
Nonetheless, great accomplishments were possible without telescopes,
e.g.:

Determination of the size and shape of the Earth and Moon (Greeks,
500 BC - 200 AD)

Discovery of Earth's motion around the Sun (Copernicus, 1513)

Kepler's Laws of Planetary Motion (1609, based on Tycho's
observations), which led to Newton's Laws of Motion and the
modern scientific revolution

The human eye is a remarkably capable
astronomical instrument. Follow this link for background information
on its function and the important observing considerations of
dark adaptation and averted vision.

B. MOTIVATIONS TO OBSERVE THE SKY

Astronomy is the "oldest science." Systematic observations of the
sky, ranging from crude to sophisticated, were made by nearly all
historical cultures, pre-literate and literate.
Initial motivations:

Curiosity---the most enduring motivation for trying to understand nature

Aesthetics

Fear/Religious Belief:

E.g.:
Astrology. This is
the idea that the motions of the Sun, Moon, and planets against the
stellar background can be used to predict the future and can influence
human personalities. It derived from the ancient belief that these
objects are living gods, who betray their intentions by their
movements. This was obviously a powerful motivation for observing the
sky. In the carving shown at the right the Egyptian Pharaoh Akhenaton
(ca. 1350 BC) and his family are communing with the Sun god Aten, the
source of Akhenaton's power.
As our scientific understanding grew, astrology lost its interest for
most people. We realized that the Sun, Moon, and planets are
inanimate objects, moving in highly regular and predictable patterns
in response to the well understood force of gravity. Constellations
and the Zodiac were recognized to lack physical significance (see
below). New planetary bodies were discovered that astrologers had
somehow failed to detect. Statistical tests showed no correlation
between "sun signs" and personality or personal history. There is no
evidence, theoretical or empirical, for astrology. Astrology lingers
only as a form of pseudo-science. But it, and related ideas, did play
an important historical role in encouraging the systematic
observations of the sky that ultimately led to the scientific
interpretation of the solar system.

Study of the sky quickly reveals the existence of regular cycles in
time of the Sun, Moon, & planets. These became the central
concern of early astronomers because of their immense practical
value for:

Navigation: on land & sea

Time Keeping

Calendar Keeping: tracking the date & seasons
For early societies (e.g. the Polynesians), these could
be critical survival technologies.

C. NAKED EYE MEASUREMENTS

Only a few types of measurements are possible with the naked eye:

1. Angular Separations

Studying the geometry of the sky by measuring angles is the most basic
form of astronomy. Apart from time tracking, this was the only
accurate quantitative measurement possible before the
advent of modern instrumentation.
Measured angles can be
all-celestial ("sky", e.g. star-to-star) or celestial-terrestrial.
They can be between different celestial objects, between a
celestial object and a reference point on Earth, or across a
celestial object.
Modern Units: Degrees, minutes, seconds of arc

Astronomers quote star brightnesses on the magnitude
scale. This scale has ancient roots. It was based originally on
simply ranking the stars by their apparent brightness as seen with
the unaided eye. Without instruments, this kind of ranking is
about the best observers can do.
Today, the scale has been quantified in terms of the light power
deposited by an individual star per unit area and tied to
telescopic measurements made with electronic detectors.
The magnitude scale is logarithmic, open-ended, and runs
"backwards" (like a sports ranking scale).
Brighter objects have smaller magnitudes.
The brightest stars are about 0 magnitude; the faintest visible to the
naked eye are about 5-6 magnitude. The brighter planets and most
familiar stars have magnitudes in the range -4 to +2.
There are only 11 stars brighter than magnitude 1 visible from
Charlottesville but 1630 stars brighter than magnitude 5.

The faintest objects yet detected (by the Hubble Space Telescope)
are 30th magnitude, or over 1 billion times fainter than visible to
eye.

The human eye can make only rough measures of magnitudes; accuracy
was only possible after the invention of photography and
electronics. Magnitudes are discussed further in the notes on Stellar Astronomy.

3. Colors, Shapes (in some cases)
4. Time

Even crude measures of angles and brightnesses, if made
systematically over days, months, or years, immediately reveal the
presence of repeating time cycles in the motions of
the Sun, Moon, and planets. As mentioned above, these were important
for their practical value. But they also showed there was order in
the universe, even if the origin of the motions was mysterious. They
provided strong intellectual stimulus for investigations of the structure of
the universe.

D. EASILY VISIBLE PHENOMENA

STARS: form the backdrop or
"reference frame" against which other objects' motions are
measured. About 2000-5000 are visible to the eye (depending on
eyesight) over the whole sky. About 1000 are visible on a dark, clear
night from a given location. The brighter stars form conspicuous
patterns which seem unchanging (to the eye).

Motion: stars move in lockstep across sky from East to
West. Patterns come back to the same location in the sky after slightly
less than 24 hours. Star locations in the sky at a given time of night
change systematically throughout the year.

Can you see all the stars that exist? NO! There are vast
numbers of stars that are invisible to the eye. With our
8" telescopes, we can see stars about 800 times fainter than the
naked eye limit---and there are 5 million of these over the whole
sky. But our star system (the Milky Way Galaxy) contains about
100 billion stars, and there are billions of galaxies!

SUN: The most obvious astronomical
object (and most important for us!). Steady brightness. Has a slow
eastward motion relative to the stars (about 1o per
day; we have to infer the Sun's position since the stars are invisible
in daylight). Returns to the same place against the star background in
one year (365.25 days).

MOON: Very bright, but much fainter than
the Sun. More rapid eastward motion relative to the stars
(about 13o per day).

Shows a drastic change in brightness and phase
(bright part as a fraction of a full circle) during each cycle, from
totally dark to fully illuminated. Takes 29.5 days to return to the
same phase (e.g. "full"). This is the cycle we have formalized in our
calendar as the month ("moonth"). There are 12 lunar cycles
per year.
Note: Moonlight was of enormous practical value before electrical
lighting was invented, so the phases of the Moon were closely followed
in earlier times.

PLANETS: Less obvious. 5 bright,
starlike objects; have a slow, complex motion relative to the
stars. In order of speed of motions (fastest to slowest): Mercury,
Venus, Mars, Jupiter, Saturn. Two of these (Mercury, Venus) are
always found relatively near the Sun; others can be up to 180 degrees
away. Although all the planets have perceptible disks in telescopes,
the naked eye cannot discern these.

Other, less conspicuous, features visible to the eye (with the modern
interpretation):

METEORS: sudden streaks of light in
sky; typical rate 5-10 per hour. Small pieces of ice or rock, burning
up in Earth's atmosphere. Often called, but are definitely not,
"falling stars." Concentrations of debris (from comets) produce
"meteor showers," e.g. the
Leonids, lasting up to several days.

STAR CLUSTERS: compact groups of stars,
all formed together. E.g. Pleiades, Hyades, M13 (at right). Since all stars in
a given cluster have the same age, clusters became the keys to
understanding stellar evolution.

DIFFUSE NEBULAE: clouds of interstellar
gas lit up by hot stars. A handful are visible without telescopes.
E.g. the Great Nebula in
Orion. These mark the birthplaces of young stars. Smaller
"planetary nebulae" mark the death of old
stars.

MILKY WAY: appears as a faint, diffuse
band of light arcing across the sky. It is the combined light of
millions of distant stars in the plane of our own Galaxy (a massive,
flattened star system), seen edge-on.

EXTERNAL GALAXIES: other star systems
like our Galaxy. 4 are visible to the eye, 2 of these are in the
northern hemisphere. E.g. the
Andromeda Galaxy---the most distant object you can see without a
telescope. It is 2 million light years from us, meaning that the
light you see from it tonight started its journey 2 million years ago.
For more discussion of this lookback effect, see
this ASTR 121 page.

Interference: sky brightness: your view of
the sky is strongly affected by background sky light, both
natural and man-made. During the day, sunlight scattered by molecules
in the Earth's atmosphere produces the "blue sky" that completely
obscures almost all other cosmic objects from our eyesight (though the
Moon is often easy to see in daylight, and you can detect Venus if you
know where to look). Likewise, near full moon, only the brightest
objects are visible in the night sky because of atmospheric scattering
of moonlight. City lights create enough local "light pollution" to
rival or exceed the effects of the full moon.

The local "horizon plane" is the (ideal, imaginary) plane
"tangent" to (just touching) the Earth at your location; you can see
objects above the plane but not below. The horizon plane sweeps
across the sky as Earth spins; it determines the rising and setting of objects.
The horizon plane is different at each location on Earth.
The Celestial
Sphere (CS) is a geometric construct used to
vizualize the positions of astronomical objects.

The celestial sphere is an imaginary hollow sphere centered on Earth.
It is shown shaded in the illustration above. Directions to key
orienting positions and to each astronomical object are imagined to be
marked on the surface of sphere.
Exactly one-half of the celestial sphere (one hemisphere) is always
above your local horizon.

The zenith is the point directly overhead on the CS. It
is equidistant from all points on the horizon.
The north and south celestial poles are the points on the CS
where the projection of Earth's rotation axis pierces the CS. These
are fixed points.
The celestial equator is the outward projection of Earth's equator to
the CS. It is a "great circle" (i.e. a circle drawn on the celestial sphere
with its center coincident with the center of the sphere).
Your meridian is the great circle on the CS that passes
through both celestial poles AND your zenith. It runs from due
North to due South, splitting the celestial hemisphere in half.
The meridian is not marked on the drawing above; instead, see this figure.

The diurnal motion: The daily spin of the Earth on
its axis produces an apparent counter-rotation of the CS and its
"attached" stars across your local sky. One complete rotation around
its axis with respect to the stars takes 23h56m (note--not quite 24
hours). The Earth rotates eastward, so the sky
appears to rotate continuously westward. Objects "rise" in the
east or "set" in the west when they cross your local horizon plane.
See the figure above.

An astronomical object is said to transit when it crosses the
meridian. At this time it is farthest from the horizon and
highest in the sky.

It was natural for people to seek deeper meaning in these remote,
silent, but majestic figures at the limit of the visible world. So,
the constellations often were given important mythological or
religious associations. These were, however, strongly
culture-dependent, and the same patterns can have very different
interpretations in different cultures.

Some associations we recognize today are very ancient, going back
to around 2000 BC (e.g. Leo the lion, Scorpio the scorpion. See the
carved stone at right; click for enlargement.) Some are new
(Microscopium). Few resemble their namesake closely.

1600+: new constellations were added to fill in blanks, mostly
in the Southern Hemisphere. E.g. Telescopium, Pyxis (compass). Elaborate &
beautiful printed atlases of classical associations appear (an
illustration of the north polar constellations from an atlas by
Cellerius is shown below).

In 1930 the International Astronomical Union established formal
boundaries for 88 constellations that cover the entire celestial
sphere. 74 are visible from Charlottesville. Click here for an
illustration of the boundaries for Orion.

Significance of the constellations:

Constellations have no physical significance. The
associations are arbitrary & man-made. Constellations are not natural
groups of stars. The fainter stars in a constellation don't
participate in the pattern. Stars in a given constellation lie near
the same line of sight from Earth but are not necessarily close
to one another in space. (Click here for an
illustration in the case of Orion.) Shapes are specific to the Earth's
location in 3-D space (a fact not recognized when ancient astrological
systems, which attached significance to the shapes, were
developed).

The stars are all moving with respect to one another, even
though the changes would not be apparent to the eye except over
thousands of years. Therefore, constellation patterns are
transitory. The changing appearance of the "Big Dipper" (part of
Ursa Major) now and 100,000 years from now is shown below. Here is an
animation of the motion of the Big Dipper stars over 200,000 years.

Modern astronomers use constellations only as a convenient
"address" to roughly locate objects in the sky.

Terminology:

The zodiac
("circle of animals") is the set of constellations through which the
Sun passes in the course of a year. The Sun's path is called the
ecliptic, and the Moon and bright planets also stay near this
path. Given the modern boundaries of the constellations, there are 13
ecliptic constellations. But in classical astronomy (and current-day
astrology) there are only 12---one for each month. The ecliptic, and
hence zodiac, is determined by the accidental orientation of the plane
of Earth's orbit. Most zodiacal constellations are faint and
uninteresting (e.g. Libra, Capricorn, Aquarius).

E.g: the star at the mouth of Canis
Major (the large dog); brightest star in the sky

Sirius ("the scorched one" in Greek) --- common name

= Alpha Canis Majoris --- Bayer listing

= 9 Canis Majoris --- Flamsteed listing

= HD 48915 --- Henry Draper Catalog listing

= BD -16 1591 --- Bonner Durchmusterung listing

= SAO 151881 --- Smithsonian Astrophysical Observatory (SAO) listing

= 0645-16 --- Right Ascension/Declination coordinate listing

Bayer's Uranometria (1603) assigned Greek letters:
alpha, beta, gamma, etc. to stars, usually in order of brightness, in
each constellation; about 1300 stars have Bayer designations.
Flamsteed (1712): numbered stars in each constellation in "Right
Ascension" (west to east) order: e.g. 61 Cygni, 40 Eri, etc.; used
today for brighter stars without Bayer designations.
SAO catalog numbers are needed to locate stars in the
database stored in your Celestron telescope computers.
Modern catalogs contain up to 100 million stars (but this is still only a
small fraction of all stars in our galaxy). Most list objects
in Right Ascension order.

There are also many catalogues of non-stellar objects,
such as nebulae, star clusters, and galaxies. The three you will most
frequently encounter are the New General Catalogue ("NGC"), the Messier Catalogue
("M"), and the The Caldwell
Catalog of Deep-Sky Objects (an updated version of the Messier
Catalogue).

Find the "North Star," Polaris (in Ursa Minor = "the Little
Dipper"). Polaris is near to, but not exactly coincident with, the
North Celestial Pole. It is not a very bright star. Easiest
method is to use the two "pointers" at the end of the bowl of Ursa
Major (the Big Dipper). See the sketch below. Alternatively, if Ursa
Major is low in the sky, you can use the stars in Cassiopeia as
pointers. See this
illustration.

Then orient yourself N/S/E/W. When you face Polaris, your right
arm is toward the East and your left is toward the West.

Find your zenith and your meridian.

Orient your sky wheel to match the time of night by
aligning the date and time tickmarks. (Note: time marked is always
Standard---not Daylight Savings--- time.) Using the wheel and pattern
recognition, "hop" to other bright stars and groups. Use brighter,
more conspicuous constellations to locate others. Practice locating
all the constellations, stars, and other features on the Manual list
for this semester.

Difficulties with using charts:

The scale of the real sky is very different from a chart

Brightnesses can't be represented well on paper. Relative
brightnesses of real stars look very different.

Poor sky transparency, city lights, or moonlight reduce visibility.

Practice using averted vision to find fainter objects:
rather than looking directly at the target, stare about 15 degrees
away, but concentrate on target location. (This puts the image on a more
sensitive part of your retina.)

After you have had time to learn the constellations, you will be
examined individually by a TA on your knowledge of the sky. You will
be expected to be able to identify 20 constellations, bright stars, or
other features of the sky.

Assignment

The Constellation Lab (Lab I) will take place on the next two
usable lab nights, starting Monday, 9/5. You must attend
one of the two sessions. Whether the Observatory will be open will be
announced on the recorded message (924-7238) by 6:30 PM. We will
also send an email alert to the class.

It would be a good idea to become familiar with the various
forecasts on the ASTR 1230 Weather Page
in planning for this and later labs.